Microsoft probably wants Yahoo to strengthen its not so hot MSN unit. It now started the whole live.com brand so that might attract a few more people.
Yahoo is a lot stronger on the web (even without including search), and are not that MS friendly (Yahoo's platforms are running Linux in most cases) and I doubt they'll start moving to MS platform.
Eventually, it all boils down to the share holders and their confidence is Yahoo. If they think Yahoo can and will succeed they won't sell. If they think otherwise, they'll sell, get their cash and go find another adventure somewhere else.